Wireless communication eliminates the burden of wiring work for wired communication of the past, and is additionally catered for usage as a technology that realizes mobile communication. For example, IEEE (The Institute of Electrical and Electronics Engineers) 802.11 may be cited as an established standard regarding wireless LANs (Local Area Networks). IEEE 802.11a/g is already widely prevalent.
With many wireless LAN systems such as IEEE 802.11, an access control protocol based on carrier sense such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is implemented, with each station being configured to avoid carrier collisions during random channel access. Also, virtual carrier sensing may be cited as a methodology for resolving the hidden terminal problem in wireless communication. The RTS/CTS handshake is a representative example of a signal transmission sequence utilizing virtual carrier sensing.
FIG. 10 illustrates a major frame format used for an RTS/CTS handshake in an IEEE 802.11 system. As illustrated, IEEE 802.11a/b/g frames are all composed of a PLCP (Physical Layer Convergence Protocol) preamble and PLCP header which correspond to a physical header, and a PSDU (PHY Service Data Unit) field which corresponds to a MAC (Media Access Control) frame. Also, FIGS. 11A to 11C illustrate respective PSDU formats for the RTS, CTS/ACK, and DATA frames defined in IEEE 802.11.
At the beginning of a PSDU, a Frame Control field and a Duration field are jointly defined. The Frame Control format is further segmentalized with various information stated therein, such as the frame type or protocol version, a resend indicator, and data path information, for example. In the Duration, a counter value called the NAV (Network Allocation Vector) is set. The counter value is taken to indicate the transmission completion time for a subsequent ACK frame, for example. A frame-receiving station to which the frame is not addressed set a NAV counter value on the basis of information in the Duration, and refrains from transmission operations during a communication sequence unit.
As illustrated in FIG. 11A, in an RTS frame, a Receiver Address (RA) indicating the recipient and a Transmitter Address (TA) indicating the sender are stated after the Duration. Also, as illustrated in FIG. 11B, in a CTS frame and an ACK frame, respective sender addresses (TAs) of an RTS and a DATA frame are copied in the Receiver Address (RA) following the Duration. Also, as illustrated in FIG. 11C, in a DATA frame, a plurality of Address fields Addr 1 to 4 are included following the Duration, and are used in order to specify the sender and recipient stations, etc. Also, net information provided to upper layers is stored in the Frame Body following the Address fields. A FCS (Frame Check Sequence) consisting of a 32-bit CRC (Cyclic Redundancy Check) is appended at the end of all frames. For example, at a recipient station that receives a frame, the FCS is recalculated and checked to determine whether or not it matches the FCS that was sent. In the case where they do not match, that frame is discarded as corrupted. In so doing, only correct MAC frames are recognized and processed.
An exemplary RTS/CTS communication sequence will be explained with reference to FIG. 12. In the figure, there exist four stations STA2, STA0, STA1, and STA3, wherein only adjacent stations are positioned within radio wave range. STA3 is a hidden terminal to STA0, and STA2 is a hidden terminal to STA1. Given such a communication environment, consider circumstances where STA0 wants to transmit information to STA1 using an RTS/CTS handshake.
Upon producing a transmit request at time T0, STA0 monitors the medium state for just a given frame interval DIFS (Distributed Inter Frame Space), and if no transmission signal exists in this space, conducts a random backoff. In the case where no transmission signal exists in this space as well, STA0 obtains an exclusive channel usage transmission opportunity (TXOP), and transmits an RTS frame to STA1 at the time T1. Herein, information indicating that the frame is an RTS is stated in the Frame Control field of the RTS frame, information indicating the amount of time until the transmission transaction related to the frame ends (i.e., the amount of time until the time T8) is stated in the Duration field, the address of the recipient STA1 is stated in the RA field, and the address of the STA0 itself is stated in the TA field.
This RTS frame is also received by STA2, the station adjacent to STA0. STA2 conducts virtual carrier sensing upon recognizing from the Frame Control field that the frame is an RTS frame and also recognizing from the RA field that the frame is not addressed to STA2 itself. In other words, STA2 recognizes that the medium is occupied until the time T8 when the transmission transaction ends, and STA2 enters a transmission-denied state without conducting physical carrier sensing. The work of entering this transmission-denied state is realized by set a NAV counter value on the basis of information stated in the Duration field and refraining from transmission operations until the counter expires, and is also called “setting a NAV”.
Meanwhile, STA1, upon receiving an RTS frame in which its own address is stated in the RA, recognizes that the adjacent station STA0 whose address is stated in the TA wants to transmit information to STA1 itself. Then, STA1 replies with a CTS frame at a time T3 after a given frame interval SIFS (Short IFS) has elapsed since the time T2 when reception of the RTS frame ended. Information indicating that the frame is a CTS frame is stated in the PSDU Frame Control field inside this CTS field. Information indicating the amount of time until the transmission transaction related to the frame (i.e., the amount of time until the time T8) is stated in the Duration field. The address of the sender (STA0) that was stated in the TA field of the RTS frame is copied to the RA field.
This CTS frame is also received by STA3, the station adjacent to STA1. STA3 conducts virtual carrier sensing upon recognizing from the Frame Control field that the frame is a CTS frame and also recognizing from the RA field that the frame is not addressed to STA3 itself. In other words, STA3 recognizes that the medium is occupied until the time T8 when the transmission transaction ends, and STA3 enters a transmission-denied state without conducting physical carrier sensing.
Meanwhile, STA0, upon receiving a CTS frame in which its own address is stated in the RA, recognizes that STA1 has acknowledged the initiate transmission request from STA0 itself. Then, STA0 initiates transmission of a DATA frame at a time T5 after a given frame interval SIFS has elapsed since the time T4 when reception of this CTS frame ended.
DATA frame transmission ends at a time T6, and in the case where STA1 is able to decode the frame without error, replies with an ACK frame at a time T7 after a given frame interval SIFS. Then, the transmission transaction for a single packet ends at a time T8 when STA0 finishes receiving this ACK frame.
When the time T8 is reached, the respective hidden terminals STA2 and STA3 drop their NAVs and return to an ordinary transmission state.
According to the RTS/CTS handshake, nearby stations STA2 and STA3 that were able receive at least one of an RTS and a CTS transition to a transmission-denied state. As a result, STA0 and STA1 are able to transmit information from STA0 to STA1 and reply with an ACK from STA1 without being impeded by sudden transmission signals from a nearby station. In other words, by using the RTS/CTS handshake in conjunction with the CSMA/CA control protocol, it may be possible to reduce collision overhead in an overloaded state.
In a conventional wireless LAN system, the CSMA/CA control protocol is not only effective for intra-network interference, but also for inter-network interference. For example, as illustrated in FIG. 13, consider the case there exist two adjacent networks, with one network being composed of STA0 acting as an access point with STA1 and STA2 connected thereto, and the other network being composed of STA4 acting as an access point with STA3 and STA5 connected thereto. With IEEE 802.11, control is conducted so as to not produce unnecessary collisions between nearby stations according to a virtual carrier sensing mechanism like that discussed above. Consequently, even if STA2 and STA3 exist within each other's signal ranges, by setting a NAV STA2 is able to avoid conditions in which STA2 receives interference due to a signal from STA3 while receiving a signal from STA0.
Meanwhile, with the IEEE 802.11a/g standard, orthogonal frequency-division multiplexing (OFDM) is used in the 2.4 GHZ band or the 5 GHz band to support a modulation method that achieves a maximum communication rate (physical layer data rate) of 54 Mbps. Also, with the standard's amendment IEEE 802.11n, MIMO (Multi-Input Multi-Output) communication methods are adopted to realize high throughput (HT) exceeding 100 Mbps. Herein, MIMO is a communication method that realizes spatially multiplexed streams by providing a plurality of antenna elements at both the transmitter end and the receiver end (as is commonly known).
For example, by increasing the number of antennas on a MIMO communication device to increase the number of spatially multiplexed streams, throughput for 1-to-1 communication can be improved while maintaining backwards compatibility. Improvements in per-user throughput for communication as well as in throughput for multiple users overall is being demanded for the future.
The IEEE 802.11ac Working Group is attempting to formulate a wireless LAN standard whose data transfer rate exceeds 1 Gbps by using the frequency band below 6 GHz. For its realization, space-division multiple access methods whereby wireless resources on a spatial axis are shared by a plurality of users, such as multi-user MIMO (MU-MIMO) or SDMA (Space-Division Multiple Access), are effective.
With a space-division multiple access system, it is possible to spatially separate multiple user signals received contemporaneously by conducting signal processing that multiplies the outgoing/incoming signals of the plurality of antenna elements by wait values. It also becomes possible to contemporaneously distribute a plurality of signals to multiple users by multiplying signals by similar wait values and then transmitting.
When starting operation of space-division multiple access with a new wireless LAN standard, it is necessary to give due consideration to backwards compatibility with the old standard, since it will be necessary to operate in a communication environment where communication devices of the new standard and communication devices of the old standard. In the legacy IEEE 802.11 standard, carrier sensing mechanisms such as CSMA/CA and RTS/CTS were adopted. Consequently, in a new standard such as IEEE 802.11ac, it is necessary to optimally combine carrier sensing and space-division multiple access.
For example, there has been proposed a communication system that combines the two technologies of carrier sensing in the legacy IEEE 802.11 standard and space-division multiple access with an adaptive array antenna by using RTS, CTS, and ACK frames in a frame format that maintains backwards compatibility with the legacy 802.11 standard (see PTL 1, for example).
FIG. 14 illustrates an exemplary transmission sequence using an RTS/CTS handshake in a space-division multiple access system. In the example illustrated, there exist three stations STA0, STA1, and STA2, and it is assumed that STA0 transmits data contemporaneously to STA1 and STA2.
STA0 conducts physical carrier sensing in advance and confirms that the medium is clear, and after additionally conducting a backoff, sends an RTS frame which indicates that STA0 will transmit information to STA1 and STA2 by space-division multiple access. However, the format of the RTS frame used at this point is not necessarily limited to that illustrated in FIG. 11A. Also, a term different from RTS may be determined by the standard.
In response to receiving an RTS frame, STA1 and STA2 contemporaneously transmit respective CTS frames (CTS-1, CTS2) in order to indicate that they are in a state able to receive information. However, the CTS frames used at this point are not necessarily that illustrated in FIG. 11B, and are assumed to be in a format enabling STA0 to separate the two signals. Also, a term different from CTS may be determined by the standard.
STA0, on the basis of the incoming signals of the received CTS-1 and CTS-2, multiplies these signals by a wait value for each antenna element required for spatial separation (i.e., conducts antenna coefficient learning), thereby separating and receiving the two signals. Additionally, STA0 uses this wait value to contemporaneously transmit DATA frames (DATA-1, DATA-2) to STA1 and STA2. DATA-1 and DATA-2 are frames transmitted by signals that are sent while taking into account the wait coefficients of the antennas such that interference does not occur at their destinations. STA1 is able to receive DATA-1, while STA2 is able to receive DATA-2.
Once STA1 and STA1 finish receiving their respective DATA frames, they contemporaneously reply with ACK frames (ACK-1, ACK-2). STA0 then receives these ACK frames, thereby ending a sequence for transmitting data to multiple stations using space-division multiple access.
Although an exemplary sequence for transmitting information by utilizing the RTS/CTS handshake is illustrated in FIG. 14, contemporaneous data delivery by space-division multiple access may also be applied to frame exchange sequences besides the above. However, since the principal matter of the present invention is not directly related to which communication sequence is used, further explanation thereof will not be given in this specification.
With a wireless LAN system of the past, inter-network interference can be avoided by a CSMA/CA control protocol as discussed above. With 1-to-1 communication, time management for securing a station's band may be comparatively loose. In contrast, with a system to which space-division multiple access has been applied, it is necessary to secure a band for all stations to be multiplexed, which demands more strictness in time management.
Hereinafter, the problem of inter-network interference in a space-division multiple access system will be examined in detail.
Assuming the transmission sequence illustrated in FIG. 14, it becomes necessary for STA0 to contemporaneously transmit data to the plurality of peers STA1 and STA2. In other words, it is necessary for STA0 to secure a state such that transmission can occur with both destinations STA1 and STA2 at the timing when transmission of a plurality of DATA frames (DATA-1, DATA-2) is initiated.
For example, in the case of assuming a station placement wherein a plurality of networks overlap as illustrated in FIG. 13, conditions are expected wherein STA0's transmission sequence and STA4's transmission sequence overlap in time and interference occurs between STA2 and STA3. At this point, in the case where STA2 has set a NAV due to receiving a CTS frame from STA3, STA2 will be unable to reply to an RTS frame addressed to STA2 itself from STA0. As a result, information is not transmitted from STA0 to STA2, and waste occurs. In contrast, assuming the case where a NAV is not set even though STA2 has received a CTS frame from STA3, an outgoing signal from STA2 will interfere with data reception at STA3, and waste will similarly occur.
Consequently, in the case where it is desired to efficiently operate a space-division multiple access system in which parts of wireless networks are placed within radio wave range of each other such that their stations interfere with each other as illustrated in FIG. 13, it is preferable to arrange transmission sequence units for each network so as to not overlap in time, as illustrated in FIG. 16, for example.
Also, FIG. 17 illustrates another exemplary station arrangement in which a plurality of networks overlap. In the example illustrated, there is a network having an access point STA0 with STA 1 and STA2 being connected thereto as terminals (client devices), and a network having an access point STA4 with STA3 and STA5 connected thereto. STA1 is within radio wave range of STA4. In this way, problems similar to the above occur even in the case where an access point STA4 is within interference range of a terminal STA1 connected to another network, and wireless network usage efficiency worsens significantly.
In this way, in the case where it is desired to conduct space-division multiple access in circumstances where exclusive placement of frequency channels used by networks is difficult, such as with wireless LAN devices, it is preferable to perform control such that transmission sequences do not overlap in time among wireless networks or among devices.
For example, proposals have been made for a system that detects the presence of a nearby network by receiving a pilot signal (see PTL 2, for example). However, in typical wireless LAN systems pilot signals do not exist and only ordinary frames are transmitted, making utilization of this technology difficult. Also, a wireless network does not actively report to nearby equipment when its own signals will be transmitted. For this reason, it cannot be arranged in advance such that signals do not overlap among networks.
Also, proposals have been made for a wireless communication system that takes a resolving action once the problem of inter-network interference becomes significant (see PTL 3, for example). However, it is desirable to control networks such that the problem does not occur in the first place.
Also, neither system disclosed in PTL 2 and 3 assumes space-division multiple access. In a space-division multiple access system, the problem of inter-network interference is significantly exhibited compared to the case of carrying out a CSMA/CA system protocol with an ordinary wireless LAN system. For this reason, the inventors reason that there is a need for a methodology that discovers and coordinates inter-network interference problems earlier.